Despite the recent and significant improvements made in Very-Large-Scale-Integration (VLSI) and VLSI circuit design, there is a continuing need for faster and more reliable devices capable of being more highly integrated. Moreover, whereas bipolar technologies have dominated high speed and high power applications in the past, Complementary Metal Oxide Semiconductor (CMOS), and more particularly Semiconductor-On-Insulator (SOI) technologies are now becoming an alternative design choice for state-of-the-art integrated circuit device applications. This is because SOI devices: (1) are inherently self-isolating and radiation tolerant, (2) have low parasitic capacitance, (3) exhibit reduced latch-up and hot-electron injection into the gate oxide and gate electrode, and (4) have improved short-channel characteristics, when compared to bulk CMOS designs. For example, these features of SOI devices are described in a publication entitled Some Properties of Thin-Film MOSFETs by Colinge, published in the IEEE circuits and Devices Magazine, pp. 16-20, November, 1987.
Furthermore, properly designed SOI field effect transistors may be integrated with conventional bipolar designs to form three-dimensional Bipolar-CMOS (BICMOS) integrated circuits. Thus, integrated circuit designers can incorporate the best features of bipolar technology with the best of SOI into a single design architecture. Three-dimensional integration, involving the vertical integration of semiconductor devices, provides higher integration densities compatible with the needs of VLSI without requiring additional device scaling.
Semiconductor-On-Insulator (SOI) can be achieved in silicon by several methods including Separation by Implantation of Oxygen (SIMOX) and Zone Melt Recrystallization (ZMR). In SIMOX, the insulating region is formed by ion implanting high energy oxygen. In ZMR, a polycrystalline silicon layer is deposited on an insulator and then recrystallized by laser or electron-beam scanning to form a quasi-monocrystalline layer. Since both of these techniques require high temperature anneal cycles to improve the quality of the SOI material, neither is compatible with three-dimensional integration or sub-micron BICMOS technologies.
Recently, new techniques for fabricating SOI structures have emerged. These techniques include Epitaxial Lateral Overgrowth (ELO) and Confined Lateral Selective Epitaxial Growth (CLSEG). Both of these techniques are compatible with BICMOS designs. For example, a publication written by Bashir, and coinventors Venkatesan and Neudeck, entitled A Novel Three Dimensional BICMOS Technology Using Epitaxial Lateral overgrowth of Silicon, published at the Custom Integrated Circuits Conference in May, 1991, describes a BICMOS structure formed using ELO techniques.
Two major impediments to error-free operation in SOI field effect transistors are the parasitic bipolar transistor latch and hot-electron injection into the gate oxide. It is known in the art that when a short-channel transistor operates in the saturation mode, a high electric field can develop between the channel pinch-off and drain junctions. This operating mode can cause the generation of impact-ionized electron-hole pairs; if the generation is regeneratively sustained, catastrophic failure ("latch-up") can result. Hot electron injection into the gate oxide, a related mechanism, can occur if the generated electrons have a sufficiently high energy ("hot") to overcome the potential barrier at the gate oxide/channel interface. Over time, repeated electron injection can cause incremental threshold voltage shifts and transconductance (g.sub.m) degradation; consequently, long term reliability is impaired.
Both mechanisms can be controlled, however, by designing transistors with lightly doped source (LDS) and drain (LDD) regions nearest the channel. Unfortunately, although techniques for controlling latch-up and hot-electron injection in bulk CMOS designs are well known, effective techniques for controlling these parasitics in SOI devices are not as well developed.
Another impediment to SOI device operation is the increased parasitic resistance that results from forming the source and drain in a thin film. Unlike conventional planar devices formed in bulk silicon, thin film devices are inherently more resistive at the source and drain because the cross-sectional area perpendicular to the direction of current flow is significantly reduced. Accordingly, it would be advantageous to develop SOI devices having enlarged source and drain areas adjacent the channel. Attempts have been made to fabricate a dual-gated SOI transistor with top and bottom gate regions formed above and below the channel. For example, in a publication entitled Double-Gate Silicon-On-Insulator Transistor with Volume Inversion: A New Device with Greatly Enhanced Performance, by Balestra, et al., IEEE Electron Device Letters, Vol. EDL-8, No. 9, pp. 410-412, September 1987, a SIMOX field effect transistor is disclosed with a top and bottom gate electrode. The structure is not suited for large scale integration, however, because the bulk silicon substrate forms a common bottom gate. Accordingly, the integration of multiple devices having independently controllable bottom gates, wherein each bottom gate is electrically isolated from one another, is not possible. In order to form a bottom gate layer separate from the substrate, two oxygen implants would be required to form a double SIMOX structure. Still unsolved, however, is the problem of electrically isolating the individual bottom gates in a manner consistent with the requirements of VLSI integration.
In another article by Adan, et al. entitled Analysis of Submicron Double-Gated Polysilicon MOS Thin Film Transistors, published at the 1990 International Electron Device Meeting (IEDM) conference, pp. 399-402, a dual-gate field effect transistor formed as a sandwich of three polysilicon layers is disclosed. The top and bottom polysilicon layers form the top and bottom gate electrodes, respectively, and the channel is formed in the middle polysilicon layer. As described in the article, the grain boundaries inherent in the polycrystalline channel create a high-density of trap states and impair performance vis-a-vis devices formed of monocrystalline material. The article also discloses a technique for self-aligning the source and drain regions to the top gate, however, the bottom gate is not self-aligned to the top gate and is specifically disclosed as a layer. The presence of this layer precludes the formation of adjacent devices having separate and electrically isolated bottom gates.
In summary, the art has yet to produce a self-aligned dual-gated silicon-on-insulator field effect transistor having isolated and independently controllable top and bottom gates, a monocrystalline thin-film channel, reduced source and drain capacitance, enlarged source and drain areas adjacent the channel and improved short channel and hot carrier immunity compatible with conventional CMOS/BICMOS applications.